1. Field of the Invention
The present invention generally relates to broadcasting networks, and more particularly to technology for providing time diversity for mobile broadcast services.
2. Related Art
A single-frequency network (SFN) is a collection of transmitters operating on the same frequency for carrying the same information to receivers in a given area. The transmitters emit identical signals, several of which may be received more or less simultaneously by individual receivers. One advantage of using multiple transmitters instead of one powerful transmitter is that multiple transmitters provide alternate paths for the signal to enter a structure, such as a house, thereby providing better reception. In mountainous areas, for example, it may be difficult to find one location capable of serving all the population centers in the area, since they are often located in valleys. Multiple transmitters can be strategically placed to cover such small areas and fill in the gaps.
One application of SFNs is for transmission of digitally encoded data such as digital television (DTV), the system and related standards for which have been established by the Advanced Television Systems Committee (“ATSC”). Under the ATSC's DTV standard (or A/53 standard), hereby incorporated herein by reference in its entirety, it is possible to transmit large amounts of data including high definition pictures, high quality sound, multiple standard definition pictures, and other ancillary related or unrelated communications, which may be accessible by using a computer or television set.
The DTV standard includes the following layers: the video/audio layer, compression layer, transport layer, and the transmission layer. At the top of the hierarchy is the uncompressed digital signal in one of the various digital data formats (e.g., video/audio formats). The data stream that corresponds with the video/audio layer is known as the elementary stream.
The compression layer compresses the elementary stream into a bitstream with a lower data rate. In the ATSC DTV standard, MPEG-2 compression is used for the video and the Dolby AC-3 compression is used for the audio. The compressed bitstream, in turn, may be packetized and multiplexed with other bitstreams into a higher data rate digital bitstream in the transport layer by an multiplexer. The MPEG-2 transport protocol defines (among several other things) how to packetize and multiplex packets into an MPEG-2 transport stream. The result is a stream of highly compressed data packets in a multiplexed bitstream which may include multiple programs and/or multiple data signals.
The multiplexed bitstream from the transport layer is modulated onto a radio frequency (RF) carrier in the transmission layer by a transmission system. The terrestrial broadcast mode utilized in the current ATSC DTV standard to transmit digital signals over the airwaves is called eight-level Trellis Coded vestigial sideband (8T-VSB).
FIG. 1 is a block diagram of a well known Trellis-coded 8T-VSB transmitter 100 used in an RF transmission system. The transmitter receives the incoming data packets of interspersed video, audio, and ancillary data, and, using a data randomizer 102, randomizes the data to produce a flat, noise-like spectrum. A Reed-Solomon (RS) encoder 104, known for its good burst noise correction capability and data overhead efficiency, RS-encodes the randomized data to add parity bytes to the end of each data packet. In turn, the data is convolutionally interleaved (i.e., spread out) over many data segments by a byte data interleaver 106.
A pre-coder and Trellis encoder 108 (referred to in the specification hereafter as a “Trellis coder”) add additional redundancy to the signal in the form of multiple data levels, creating multilevel data symbols for transmission. A synchronization insertion component 110 multiplexes the segment and frame synchronizations with the multilevel data symbols before a DC offset is added by a pilot insertion component 112 for creation of the low-level, in-phase pilot. Segment and frame synchronizations are not interleaved. A VSB modulator 114 provides a filtered intermediate frequency (IF) signal at a standard frequency, with most of one sideband removed. Finally, an RF upconverter 16 translates the signal to the desired RF channel.
Multipath propagation is a common problem in single transmitter broadcast environments because it places a burden on a receiver equalizer's ability to handle signal echoes. In a distributed transmission system, where multiple transmitters are utilized, the multipath propagation problem is compounded. It is necessary, therefore, to synchronize or adjust the timing of the SFN system to control the delay spread seen by receivers in areas of SFN induced multipath not to exceed delay handling range of receiver equalizers and become problematic.
In addition, the output symbols of each transmitter is based on the transport stream received, how this is then mapped into a Data Frame and the initial states of the Trellis coders, which are normally random. When the transmitters emit the same symbols as one another for the same data inputs, they are said to be made “coherent”. If the transmitters in an SFN are not synchronized, they will not emit coherent symbols.
The ATSC has promulgated a standard, referred to as the A/110 standard, which provides rules for synchronization of multiple transmitters emitting Trellis-coded 8T-VSB signals in an SFN or distributed transmission system (DTx) to create a condition which allows multiple transmitters being fed by the same transport stream to produce coherent symbols. SFN and DTx are to be understood to be synonymous terms. The A/110 standard is hereby incorporated herein by reference in its entirety.
Another standard promulgated by the ATSC, referred to as the A/111 standard, provides ATSC recommended practices for the design of synchronous multiple transmitter networks. The A/111 standard is also hereby incorporated herein by reference in its entirety.
FIG. 2 shows a block diagram of an ATSC SFN system 200 using A/110 distributed transmission (DTx). SFN system 200 includes three elements: an external time and frequency reference (shown as GPS), a distributed transmission adapter (DTxA) 202 situated at the source end of the distribution (or studio-to-transmitter link (STL)) subsystem, and plural RF transmission systems 208. DTxA 202 includes two basic blocks: a transmitter synchronization inserter 206 and a data processing model 204. Transmitter synchronization inserter 206 inserts information (described in more detail below) into the transport stream (TS). The data processing model 204 is a model of the data processing in an ATSC modulator which serves as a master reference to the slaved synchronized data processing blocks 210 in the RF transmission systems 208. Generally, each RF transmission system 208 includes two blocks: synchronized data processing block 210 and signal processing and power amplification block 211, which collectively are sometimes referred to as a “modulator” 212. These low level stages of the transmitter are also generally referred to as the “exciter” component. Herein the terms exciter and modulator are use interchangeably.
In an ATSC SFN system each synchronized data processing block 210 also includes a Trellis-coded 8-VSB transmitter 100 discussed above with reference to FIG. 1. As shown in FIG. 2, the DTxA produces a transport stream (TS) and feeds this stream to all of the synchronized data processing blocks 210.
FIG. 3 shows the structure 300 of a distributed transmission packet in accordance with the A/110 standard and FIG. 4a depicts a VSB data frame, which includes packets of data and forward error correction (FEC), and data field synchronization (DFS) fields.
The A/110 standard requires the following three ATSC system elements to be synchronized: 1. frequency synchronization of the pilot or carrier frequencies, 2. data frame synchronization, and 3. pre-coder and Trellis encoder (Trellis coder) synchronization. A description of how these three elements are synchronized in a group of separately located transmitters follows.
According to the A/110 standard, control of two specific transmitter frequencies is required. First the RF frequency of the transmitted signal, as measured by the frequency of its pilot, must be accurately controlled to maintain frequencies of the transmitters close enough to one another that the receiver is not over-burdened with apparent Doppler shift between the signals. The symbol clock frequency must be accurately controlled to allow the output symbol stream to maintain stable, relative, time offsets between transmitters in a network. A flag, stream_locked_flag, in the DTxP packet structure is used to identify one of two options for performing symbol frequency synchronization. This flag is a 1-bit field that indicates to a slave transmitter whether it is to lock its symbol clock frequency to the incoming transport stream clock frequency (normal ATSC methodology) or to lock its symbol clock frequency to the same external precision reference frequency used throughout the network (e.g., GPS).
Data frame synchronization requires all of the slave modulators 212 in an SFN to use the same transport stream (TS) packet to start a VSB data frame (FIG. 4a). In the current ATSC A/110 standard, this is accomplished by using DTXA 202 by inserting a cadence signal. In particular, a cadence signal (CS) is inserted at a deterministic point in time, once every 624 packets, into the MPEG-2 transport stream from the DTxA to each of the modulators 212. Dividing the rate of CS by half produces a Data Field Sync (DFS). The A/53 standard specifies that the data randomizer 102, RS encoder 104, and data interleaver 106 and intra-segment interleaver in part of 108 in the slave synchronized data processing blocks 210 shall all slave to DFS.
In addition, the A/110 standard provides that it is necessary to develop a state condition for the Trellis coder memories to be applied at a specific epoch in the data stream simultaneously by all RF transmission systems 208 in a network. According to the A/110 standard, “in order to put the pre-coders and trellis encoders of all the transmitters in a network in the same state at the same time, it is necessary to ‘jam sync’ them to the trellis coder model in the Distributed Transmission Adapter.” In other words, Trellis coders cannot be synchronized by identifying an epoch in the transport stream (TS). Instead, to place the Trellis coders of all the transmitters in a network in the same states at the same time, a sample of all Trellis coder states in the data processing model 204 is captured, and this data is carried in an element of the DXP, Trellis_code_state (FIG. 3), from DTxA 202 to all the slave modulators 212.
At a later, deterministic point in time, the Trellis code states that have been extracted from the DXP are used to initialize the memory of each Trellis coder in the slave modulators 212, to the state of the data processing model 204 in DTxA 202. Once this has been performed, the modulator Trellis coders are synchronized and all the modulators 212 should produce “coherent symbols.” In addition, the DTxA indicates operating mode to the transmitters and provides information to be transmitted in the data field sync data segment through a field rate side channel, which carries information updated regularly at a data field rate.
The ATSC A/110 standard further introduces an RF watermarking technique to support different types of measurements required in distributed transmission networks for setup and maintenance. RF watermarks can be carried by ATSC 8-VSB signals and used to identify transmitters and for measuring various characteristics of the signals when they are received. For example, RF watermarking permits each transmitter to simultaneously broadcast site-specific data, such as status and telemetry, to a distributed transmission network (DTxN) field monitoring or data collection point. Return channels provide a pathway to return data from one or more transmitters without requiring separate telemetry channels. Each distributed transmitter transmits an independent data signal on the broadcast channel using a Code Division Multiple Access (CDMA) technique. The return channel is a low data rate channel. However, there are no restrictions on the data it can carry.
FIG. 4b depicts a VSB data frame including CDMA channel RF watermarking injection in accordance with the ATSC A/111 standard. As shown in FIG. 4b, in addition to symbol synchronization, the RF watermark is also time-synchronized to the data frame of the 8-VSB host signal.
Generally, RF watermark signals appear to be random noise. The pseudorandom noise (PN) sequences used, however, are deterministic periodic sequences that can be detected by special-purpose receivers. The particular form of PN sequences used in the RF watermark are known as Kasami sequences, which are generated by combining the outputs of several linear feedback shift registers. The resulting RF watermark symbols are added to the host 8-VSB symbols before they are passed to the digital-to-analog conversion process of the 8-VSB modulator. This produces a low-level binary RF watermark signal riding on and synchronized with the 8-VSB host signal. A more detailed description of the aforementioned RF watermarking technique can be found in the A/111 standard, which as stated above is incorporated herein by reference.
The method used by A/110 standard to achieve Trellis coder synchronization adds much complexity to the overall SFN distributed transmission system design by requiring the DTxA 202 to sample the data processing model's Trellis coder states. Moreover, the A/110 does not provide the ability to post process data in the modulator once it exits the DTxA. A change of one bit in data stream after DTxA will break the Trellis code synchronization scheme thus making it difficult, if not impossible, to add enhancements to ATSC standard A/53. Moreover, as more transmitters are added in a multi-tier (e.g., distributed-translator) scheme the complexity of an SFN under the A/110 standard grows since an additional data processing model 204 must be added for each tier. Thus, what is needed is a technology that is scalable in SFN applications without adding additional complexity or constraints on system extensibility of the overall system.
In addition, in response to broadcasters' calls for flexible use of digital TV spectrum, the ATSC has requested the industry to explore “enhancements” to the VSB scheme that lies at the core of its DTV standard. Such enhancements include enhanced reception on fixed/indoor devices, handheld (i.e., portable) and mobile devices, and broadcasting of both video and data to mobile communicators and other handheld equipment, and the like. Heretofore, there has not been proposed in response to the ATSC request a solution which advantageously utilizes the deterministic nature of the ATSC system. Nor has the CDMA spread spectrum technique used in the current ATSC specifications been utilized to provide signaling information to receiving devices, such handheld/portable, mobile, fixed/indoor, to notify such devices of service availability in advance of service transmission. Nor have third party communication links been utilized to provide signaling information. Accordingly, another need exists to provide enhancements to both single transmitter networks as well as SFN which leverage the deterministic frame structure incorporated in these broadcast systems.
Signal coverage is always a concern to broadcasters. As described above, many mobile users reside in areas that are substantially terrain shielded from primary transmitter locations. While placing additional transmitters or boosters may help in many situations, it may not always be a viable solution to do so, such as under overpasses or in relatively short tunnels. When a mobile receiver moves through such a terrain shielded reception environment, signals fade or drop out temporarily.
Consumers demand high quality reception without substantial signal loss. The auto industry, for example, requires system availability of greater than 99.5%. This means that signal reception, must be near perfect, even under adverse conditions. There is a need therefore to mitigate the effect of such the aforementioned signal fades and dropouts.
Given the foregoing, what is needed is a system, method and computer program product for apparatus, systems and methods for providing time diversity for mobile broadcast services.